From 79b484a0275740ed94192adcec147c214f4189cd Mon Sep 17 00:00:00 2001 From: Srikar Dronamraju Date: Sat, 21 Aug 2021 14:54:17 +0530 Subject: [PATCH] powerpc/smp: Fix a crash while booting kvm guest with nr_cpus=2 Aneesh reported a crash with a fairly recent upstream kernel when booting kernel whose commandline was appended with nr_cpus=2 1:mon> e cpu 0x1: Vector: 300 (Data Access) at [c000000008a67bd0] pc: c00000000002557c: cpu_to_chip_id+0x3c/0x100 lr: c000000000058380: start_secondary+0x460/0xb00 sp: c000000008a67e70 msr: 8000000000001033 dar: 10 dsisr: 80000 current = 0xc00000000891bb00 paca = 0xc0000018ff981f80 irqmask: 0x03 irq_happened: 0x01 pid = 0, comm = swapper/1 Linux version 5.13.0-rc3-15704-ga050a6d2b7e8 (kvaneesh@ltc-boston8) (gcc (Ubuntu 9.3.0-17ubuntu1~20.04) 9.3.0, GNU ld (GNU Binutils for Ubuntu) 2.34) #433 SMP Tue May 25 02:38:49 CDT 2021 1:mon> t [link register ] c000000000058380 start_secondary+0x460/0xb00 [c000000008a67e70] c000000008a67eb0 (unreliable) [c000000008a67eb0] c0000000000589d4 start_secondary+0xab4/0xb00 [c000000008a67f90] c00000000000c654 start_secondary_prolog+0x10/0x14 Current code assumes that num_possible_cpus() is always greater than threads_per_core. However this may not be true when using nr_cpus=2 or similar options. Handle the case where num_possible_cpus is smaller than threads_per_core. Cc: linuxppc-dev@lists.ozlabs.org Cc: Aneesh Kumar K.V Cc: Nathan Lynch Cc: Michael Ellerman Cc: Ingo Molnar Cc: Peter Zijlstra Cc: Valentin Schneider Cc: Gautham R Shenoy Cc: Vincent Guittot Fixes: c1e53367dab1 ("powerpc/smp: Cache CPU to chip lookup") Reported-by: Aneesh Kumar K.V Debugged-by: Michael Ellerman Signed-off-by: Srikar Dronamraju --- arch/powerpc/kernel/smp.c | 2 +- 1 file changed, 1 insertion(+), 1 deletion(-) diff --git a/arch/powerpc/kernel/smp.c b/arch/powerpc/kernel/smp.c index f2abd88e0c254d..91dcfc5a8de347 100644 --- a/arch/powerpc/kernel/smp.c +++ b/arch/powerpc/kernel/smp.c @@ -1109,7 +1109,7 @@ void __init smp_prepare_cpus(unsigned int max_cpus) } if (cpu_to_chip_id(boot_cpuid) != -1) { - int idx = num_possible_cpus() / threads_per_core; + int idx = max((int)num_possible_cpus() / threads_per_core, 1); /* * All threads of a core will all belong to the same core,